1. Field of the Invention
This invention relates to a method for measuring overlay between two patterned thin films using Moire fringes.
2. Brief Description of the Prior Art
As geometries of components of semiconductor devices continually decrease, fabrication techniques must also continually be altered to accommodate these smaller geometries. Among the fabrication techniques affected by this decrease in geometries is the alignment between layered thin film patterns. Accordingly, the methods used to measure the alignment or overlay between two patterned layers must become more precise and accurate.
The current technique for measuring overlay between two) patterned thin films in the fabrication of semiconductor devices is to measure the displacement between the centers of two different sized patterned boxes or frames, each box or frame created in different layers and sprinted concentrically. One box or frame is typically photoresist and the other box or frame has typically been etched into the substrate. A measurement of the displacement of the centers of the boxes or frames is a measure of how well the patterns in one thin film layer are overlayed with another thin film layer.
Moire patterns are well known in the scientific literature. Moire patterns are produced when the lines of overlapping figures cross at an angle, generally of less than 45 degrees. The Moire lines are then the locus of points of intersection. At crossings of less than 45 degrees a Moire pattern of equispaced lines is observed which are known as Moire fringes. The spacing of the fringes increases with decreasing crossing angle.
Measurement of Moire fringes has been applied to the alignment of a wafer and a reticle in lithographic applications as described by B. P. Singh, T. Goto, et al., "Tracking and dynamic control of the angular alignment position in a photolithographic mask aligner by the moire interference technique", Review of Scientific Instruments, 66 (3), 2658, (March, 1995) and in a publication of M. C. King and D. H. Berry, Applied Optics 11, page 2455 (1972). The prior art for measuring box or frame type targets is set forth in a paper of M. Davidson, K. Kaufman and I. Mazor entitled "First Results of a Product Utilizing Coherence Probe Imaging for Wafer Inspection", SPIE Proceedings, Vol. 921, 1988 which discloses little more than automated microscopes. These types of tools can be modified for detection and measurement of the Moire fringe pattern. Visual inspection can also be performed using microscopes if some type of graduated scale is patterned in addition to the gratings.